Microelectronic devices often include a layer of conductive material on or proximate a surface of the device to facilitate electrical connections to the device. The layer of conductive material typically includes several interconnect structures and is often referred to as an interconnect layer or level because it is configured to couple one or more conductive or semiconductive layers within the device to an apparatus external to the device such as an electronic package, a substrate, or the like.
Interconnect layers typically include a thin, patterned layer of aluminum or aluminum copper (having less than about 5% copper) and are formed by depositing the aluminum onto a surface of the device, patterning the aluminum with photoresist, and etching the patterned aluminum to form the interconnect structures. Dielectric material such as silicon oxide, silicon nitride; low dielectric constant materials such as BCB, SiOF, SILK, or FLARE; or a combination of any of these materials is then deposited over the interconnect structures and vias are formed through the dielectric material to allow electrical connections to the interconnect structures.
Electrical connection to the interconnect structure is often formed by attaching one end of a thin metal wire to the interconnect structure and the other end of the wire to an external apparatus such as a substrate, a leadframe, or the like; this process is referred to as wire bonding.
Wire bonding typically includes placing the wire in contact with the aluminum interconnect structure (typically at an elevated temperature) and moving the wire relative to the structure at high speed (e.g., by applying ultrasonic vibration to the wire) to bond the wire to the interconnect structure. Application of ultrasonic vibration assists in bonding the wire to the interconnect structure by, among other things, moving the wire across the surface of the interconnect structure to break aluminum oxides that form on the surface of the structure, allowing the wire to bond to the aluminum metal. Also, moving the wire relative to the interconnect surface assists chemical and mechanical bond formation between the wire and the structure.
Although other methods and apparatus may be used to form electrical connections to the device, wire bonding is often preferable because, among other reasons, the technology is well understood and many device packagers already have wire bonding equipment in place, making wire-bond based packaging relatively inexpensive and reliable. Accordingly, methods and apparatus for forming electrical connections to a microelectronic device may desirably include wire bonding.
As the size of microelectronic devices decreases (e.g., to increase device speed or increase the number of devices per surface area of a wafer on which the devices are formed) aluminum interconnect structures become increasingly less desirable. In particular, as device size and corresponding interconnect structure sizes are reduced, an amount of current per unit area (current density) that each structure must transport increases, and as the current density increases, electromigration of the aluminum structures increases. In addition, the resistance in the structures increases as the structure size decreases, causing the devices to heat during use and reducing the performance of the device (e.g., by increasing power consumption of the device).
Recently, copper interconnect structures have been developed to overcome many of the shortcomings of aluminum interconnect structures. Use of copper to form the structures is advantageous because copper has higher resistance to electromigration and lower electrical resistance compared to aluminum. However, integration of copper processes in microelectronic formation has been problematic. In particular, wire bonding to the copper interconnect structures has been arduous.
Forming wire bonds to copper interconnect structures is difficult, in part, because unlike aluminum oxides that form on the surface of aluminum interconnect structures, copper oxides do not readily break during the wire bonding process to allow the wire to bond to the structure. Rather, the copper oxides often act as a lubricant when the wire is placed in contact with and moved relative to the interconnect structure, making bonding of the wire to the copper structure difficult.
Prior art methods for improving bonds between the copper interconnect and the wire include coating a surface of the copper interconnect structure with a metal such as aluminum or combinations of nickel and gold. Although these materials may provide a suitable surface for the wire to attach, the metals are generally less conductive than copper and require additional device manufacturing steps to deposit and pattern the metals on the wafer surface. Using metals that are less conductive than copper mitigates at least some of the advantages associated with using copper to form the interconnect structure such as increased device speed and lower power consumption, and using additional processing steps undesirably increases fabrication costs associated with manufacturing the microelectronic devices. Accordingly, microelectronic devices including improved interconnect structures and methods for forming the structures are desirable.